1. Field of the Invention
The present invention relates to a novel solder for a can body and particularly to a tin solder composition for soldering the side seam of a tinplate can body having improved resistance to sulfur caused discoloration.
2. Description of the Prior Art
Metal containers for the packaging of food products are generally comprised of a tinplate body having two of its marginal edges interfolded by means of hooks to form a side seam. In the manufacture of the food cans, the can body is made by forming an enamel coated, flat tinplate blank into substantially tubular configuration, reversely bending the longitudinally extending margins thereof to form outer and inner reversely bent body hooks respectively, inter-engaging the body hooks, bumping the engaged hooks to mechanically lock them together and then flowing molten solder into the interstices between the locked body hooks whereby upon cooling, the solid solder bonds and hermetically seals the side seam. By the application of the solder, a joint is formed in which the solder extends into and around the body hooks and forms a thin line of solder on the inside of the can. After application of the solder, the side seam area is coated with an enamel coating.
the solder presently in general use for tinplate food containers is composed of a tin alloy of 98% lead and 2% tin. U.S. Pat. No. 3,268,344 discloses a solder for tinplate food containers composed of 100% tin.
The metal can industry has continually sought to improve the ability of soldered tinplate cans to withstand corrosive attack by a product packed therein in order to prevent the can from releasing undesirable contaminants into the food product. It is known that one of the more troublesome areas subject to corrosive attack and to contaminant formation is the side seam area of the can body; and to a large extent this corrosion susceptibility is due to blackening due to the formation of iron sulfide particles, which tend to degrade the appearance and the marketability of the can and product packed therein. Such blackening is caused by degradation products of sulfur-containing protein-rich foods such as meat and fish in contact with the soldered side seam area chemically attacking the area. Accordingly, there has been and still is a quest in the metal can industry to improve the resistance of soldered food cans to sulfur caused discoloration.